This invention relates generally to a method and apparatus for continuous casting of thin slabs and more particularly to a continuous casting method and apparatus wherein molten metal flows horizontally as it solidifies into the thin slab.
Conventional continuous casting machines for casting of ferrous and non-ferrous slabs are of the vertical type. Molten steel is introduced at the top of the mold and flows downwardly to the bottom. As the molten steel contacts the relatively much cooler walls of the mold a solid crust or shell forms around a molten interior of the steel, the crust increasing in thickness as the steel travels downwardly. The emergent slab at the bottom of the mold will be a solidified slab of intended shape.
In prior art casting machines the cross section of the cooling mold is constant along the entire length of the mold and the finished product conforms to the shape and cross-section of the initial slab. For relatively thick slabs, for example those exceeding 6 inches in thickness, it is not particularly difficult to initiate and maintain the casting operation. However, the introduction of molten steel into a mold for generating relatively thin slabs, for example 1 to 6 inches in thickness, posses several problems including the tendency of the metal to freeze within the opening and thus stop the casting process. Submerged nozzles do not solve the problem because they still tend to freeze between the mold surface and the nozzle. The problem is aggravated in that all the sides that define the opening of the mold, act as cooling surfaces resulting in the immediate formation of a shell around a molten interior.
Another recurring difficulty with conventional continuous casters is the tendency of the molten interior aggregate to shrink across the thickness toward the cooler outer skin. This condition often results in voids or "pipes" formed at the center of the casting, producing a defective product. Furthermore, all continuous casters of the vertical type have the inherent disadvantages that the machine requires a large overhead clearance and correspondingly large overhead structure to accommodate the vertical path of the solidifying slab. Efforts have been made to reduce the required height by curving the bottom of the mold to follow a partially horizontal path. However, problems remain in that owing to the curved mold, the solidified slab has a curvature that must be straightened. If the slab is somewhat thick, for example 8 inches or thicker, the slab must be reheated, a proposition which is both expensive and complicated.
Still another difficulty with prior art continuous casting cooling molds, arises in that to give the slab a desired shape the interior peripheral walls in the mold are comprised of copper plates which are bolted to supporting outer steel structures which rigidify the copper plates and prevent thermal distortion. Coolant, for example water, is circulated over the outside of the copper plates to keep the copper plates at a relatively low temperature whereby the molten metal within the mold cools and solidifies as previously described. The efficiency of the heat removal and the concurrent formation of the skin or shell for the solidifying slab depends on the intimacy of contact between the molten steel and the interior surface of the copper plates. Further, as the slab emerges from the cooling mold with its interior at least partially molten, the outer skin must have sufficient thickness to prevent break-out of liquid steel as the slab travels through a so-called roller apron section, immediately following the cooling mold. However, as the solidifying skin of the slab is increasingly cooled, it begins to shrink in cross section and tends to separate itself from the inner surface of the copper plates. Consequently, a clearance forms between the slab and the walls of the cooling mold which greatly reduces the heat transfer efficiency in the mold due to the air gap present between the slab and the interior surfaces of the mold. Thus, the slab, at times, emerges from the mold with a shell or skin so thin that break-out of liquid from the interior and damage to the slab is possible.